1. Field of the Invention
The present invention relates to a read-only memory or ROM and to a corresponding method of manufacture by means of MOS technology.
2. Discussion of the Related Art
Different types of ROM cells are known in the prior art. These cells are based on a MOS transistor. A specific step in the manufacturing method will modify this transistor to obtain a difference in operation between the basic cell and the programmed cell to which the particular step has been applied.
There is a known method of programming cells by forming metal contacts on the gates of the transistors which are selected to be programmed. The gates on the non-programmed cells have no such contacts. It is generally this type of cell that is found in programmable logic arrays. The encoding, i.e., programming of a cell, can be easily detected since it is enough to look at where the metal (aluminum) contacts are placed. Yet, the problem of such a programming method is that the programmed cell takes up a relatively large amount of space (because of the gate metal contacts).
There are other known methods of programming cells that consist of acting on one of the diffusions of the transistor to modify the properties of conduction of these diffusions.
In one such method, there is N.sup.- encoding (programming) (on P type substrate) of the cell, where the non-programmed cell includes a normally doped (N.sup.+) diffusion and a diffusion with lower doping (N.sup.- doping). In the encoded or programmed cell, there is only the normally doped N.sup.+ diffusion.
In another such method, the basic non-programmable cell has a diffusion with low doping (N.sup.-) as above, but the encoded cell has a diffusion with an opposite type of doping (P type) between this diffusion with low doping (N.sup.- type doping) and the channel. This is the encoding by P type counter-doping (for a P type substrate).
All these methods that act on the diffusions have the major drawback of not being able to offer a sufficiently wide differentiation of the currents between the basic non-programmed cell and the encoded or programmed cell. The conductive cell (the basic non-programmed cell in the above description) shows little conductivity, owing to the low doping of the diffusions. On the other hand, the encoded or programmed cell, lets through a leakage current. For these reasons, in order to ensure detection of an encoded cell from a non-encoded cell, it is necessary to have circuits that are specially designed to detect small current differences, the response time of which detection circuits remains very lengthy.
It is also sought to shorten the manufacturing times. It is necessary that the encoding part, which is specific to the customer, should take place as late as possible in the manufacturing process. Also, the smaller the number of manufacturing steps that remain to be performed, the more speedily will the circuit be delivered to the customer. At present, starting from the time when the circuit is ready to be encoded, six to eight weeks are needed to finish a circuit according to the customer's encoding specifications. It is being sought to halve this encoding time so as to shorten the time of delivery of the encoded circuit.